1. Field of the Invention
The present invention is directed to the protection of a system's data lines connected to a MOS integrated circuit by use of switched body MOS transistors as pullups or pulldowns on the integrated circuit for driving the system's data lines.
2. Discussion of the Prior Art
As shown in FIG. 1A, an input of a conventional junction isolated MOS integrated circuit is clamped to ground or power supply potential by parasitic bipolar devices when the integrated circuit's power supply or ground is lost, respectively. This can have disastrous effects when the input is connected to the system data bus, since the entire system can be shut down because of the power or ground failure to one device. As shown in FIG. 1B, a similar problem arises in the case of an MOS output driver. FIGS. 1C and 1D show a cross-sectional view of a conventional input/output device implemented in P-well and N-well technology, respectively, revealing these parasitic bipolar devices. Note that the transistor bodies are hard wired with P-wells tied to V.sub.SS and N-wells tied to V.sub.DD.
In the conventional designs shown in FIGS. 1A and 1B, if the IC's V.sub.DD power supply is lost by being open, the IC's internal V.sub.DD bus drops due to the IC's I.sub.DD current through the chip's logic to V.sub.SS. Also V.sub.DD can be shorted to ground. In either case, when a component external to the IC attempts to pull the data line to a valid high state, this is prevented by the forward biased diode between the I/O pad and the V.sub.DD pad (at 0 volt) on the IC.
Conversely, if the IC's V.sub.SS ground is lost by being open, the IC's internal V.sub.SS bus rises due to the IC's I.sub.DD current through the chip's logic to it's internal V.sub.SS bus. When a component external to the IC attempts to pull the data line to a valid low state, this is prevented by the forward biased diode between the V.sub.SS pad (at V.sub.DD volts) and the I/O pad on the IC.